Electroluminescent (EL) devices using organic luminescent materials are being actively researched recently because of the ability of displays fabricated using EL devices to exhibit wider viewing angle and faster response time than conventional liquid crystal displays. More particularly, flat panel displays fabricated using EL devices made from organic luminescent materials are expected to use spontaneous light emission and the resultant flat panel displays have high response speed no matter what the vision angle is. Furthermore, EL devices using organic luminescent materials can exhibit advantages such as low power consumption, high brightness, and light and thin design, which can be useful in consumer electronic devices such as digital cameras, personal digital assistants (PDA), and videophones.
An example of a light-emitting device is an organic light-emitting diode (OLED) device. In general, an OLED device, which can include an organic thin film containing a luminescent material formed between an optically transparent anode and a metallic cathode, emits light when an external voltage is applied to the luminescent material. To produce a full-color EL display panel using OLEDs, it is useful to have highly efficient red, green, and blue EL materials with appropriate chromaticity and luminance efficiency.
OLEDs exhibiting high luminance efficiency can be fabricated using electroluminescent materials containing heavy metal complexes, and the electroluminescent materials attract attention in applications and researches. For example, electroluminescent materials comprising complexes of platinum (Pt), osmium (Os), and iridium (Ir) can be used to form an electroluminescent layer in OLEDs, wherein the iridium complexes exhibit the highest efficiency. Iridium complexes exhibiting high luminance efficiency typically have an octahedral structure with the iridium center in a +3 oxidation state. The mechanism of luminance emission of these iridium complexes is based on a triplet-3MLCT (metal to ligand charge transfer) transition between the metal and the ligand, or a triplet-3π-π* ligand-centered luminescence. The strong spin-orbit coupling of the heavy metal complexes produces high phosphorescence efficiency.
One of the best known triplet-state blue phosphorescent light-emitting material is Iridium(III)bis(4,6-difluorophenylpyridinato)picolate (FIrpic), and its external quantum efficiency can achieve approximately 10% (or 10 lm/W) in some reports. However, the blue light of FIrpic is not enough saturated, the CIE (Commission International D'Eclairage) chromaticity of which is (0.17, 0.34), so the light of FIrpic is merely defined as cyan or greenish blue.